Re: Eager aggregation, take 3

Richard Guo <guofenglinux(at)gmail(dot)com> 于2024年8月21日周三 15:11写道：

> On Fri, Aug 16, 2024 at 4:14 PM Richard Guo <guofenglinux(at)gmail(dot)com>
> wrote:
> > I had a self-review of this patchset and made some refactoring,
> > especially to the function that creates the RelAggInfo structure for a
> > given relation. While there were no major changes, the code should
> > now be simpler.
>
> I found a bug in v10 patchset: when we generate the GROUP BY clauses
> for the partial aggregation that is pushed down to a non-aggregated
> relation, we may produce a clause with a tleSortGroupRef that
> duplicates one already present in the query's groupClause, which would
> cause problems.
>
> Attached is the updated version of the patchset that fixes this bug
> and includes further code refactoring.
>
>
I continue to review the v11 version patches. Here are some my thoughts.

1. In make_one_rel(), we have the below codes:
/*
* Build grouped base relations for each base rel if possible.
*/
setup_base_grouped_rels(root);

As far as I know, each base rel only has one grouped base relation, if
possible.
The comments may be changed to "Build a grouped base relation for each base
rel if possible."

2. According to the comments of generate_grouped_paths(), we may generate
paths for a grouped
relation on top of paths of join relation. So the ”rel_plain" argument in
generate_grouped_paths() may be
confused. "plain" usually means "base rel" . How about Re-naming rel_plain
to input_rel?

3. In create_partial_grouping_paths(), The partially_grouped_rel could have
been already created due to eager
aggregation. If partially_grouped_rel exists, its reltarget has been
created. So do we need below logic?

/*
* Build target list for partial aggregate paths. These paths cannot just
* emit the same tlist as regular aggregate paths, because (1) we must
* include Vars and Aggrefs needed in HAVING, which might not appear in
* the result tlist, and (2) the Aggrefs must be set in partial mode.
*/
partially_grouped_rel->reltarget =
make_partial_grouping_target(root, grouped_rel->reltarget,
extra->havingQual);

--
Thanks,
Tender Wang